1. Field of the Invention
This invention relates to a nonvolatile storage device with a plurality of unit nonvolatile storage devices layered therein and a method for manufacturing the same.
2. Background Art
Nonvolatile memories, such as NAND flash memories, are widely used as high-capacity data storage in, e.g., cellular phones, digital still cameras, USB (Universal Serial Bus) memories, and silicon audio players, and continuing to expand the market as the manufacturing cost per bit is reduced by rapid downscaling. Furthermore, novel applications have also been fast emerging, achieving a virtuous circle in which downscaling and manufacturing cost reduction find new markets.
In particular, a NAND flash memory substantially realizes cross-point cells by allowing a plurality of active areas (AA) to share a gate conductor (GC), and is being rapidly downscaled because of its simple structure. Hence, recently, NAND flash memories have led semiconductor scaling, and the minimum processing dimension has reached 50 nm or less even in mass production
However, a NAND flash memory is based on the operation of a transistor which records information using its threshold voltage variation, and reportedly has limitations on further improvement of its characteristics uniformity, reliability, operating speed, and bit density. Thus, development of new nonvolatile memories is desired.
In this context, for example, phase change memory elements and resistance change elements are operated using the variable resistance of resistance materials, and hence need no transistor operation for program/erase operation. Furthermore, the device characteristics are improved as the resistance material is downsized. Thus, they are promising for realizing higher characteristics uniformity, reliability, operating speed, and integration density to meet future requirements.
Phase change memories and resistance change memories have an advantage of being easily downscaled because a plurality of memory layers can be stacked, and several memory elements having such configuration have been proposed. In these memories, unlike NAND flash memories, sensing is based on the amount of current. Hence, in phase change memories and resistance change memories, in order to avoid the sneak current during recording/reproduction, each memory cell is often provided with a rectifying element, such as a diode, for regulating the direction of current, and several manufacturing methods therefore have been proposed (e.g., JP-A-2008-034809).